1. Field of the Invention
The present invention relates to a direct-modulation frequency modulator using a PLL (Phase Locked Loop).
2. Description of the Related Art
1, 3, 4. A frequency modulator, which generates a modulated signal frequency-modulated based on an audio signal (to be modulated) to be transmitted, is used in FM broadcast and in-car audio systems. For example, a patent document 1 discloses a direct-modulation frequency modulator that includes a PLL configured using a voltage controlled oscillator (hereinafter, referred to as a VCO), a frequency divider, a phase comparator, and a loop filter and multiplexes an audio signal as a modulating signal with an input signal to the VCO.
As disclosed in patent document 1, in the conventional direct-modulation frequency modulator, a technique, which couples the modulating signal using a coupling capacitor or a resistor for voltage addition between the output terminal of the loop filter and the input terminal of the VCO, has been adopted.
2. Further, a frequency modulator that generates a modulated signal frequency-modulated based on an audio signal (modulating signal) to be transmitted is used in FM broadcast and in-car audio systems. A stereo modulator, which converts a stereo audio signal containing a left-channel signal (hereinafter, also referred to as an L-channel signal) and a right-channel signal (hereinafter, also referred to as an R-channel signal) into a stereo composite signal using a subcarrier wave and a pilot signal, is provided upstream of the frequency modulator. For example, patent documents 2 to 4 disclose such stereo modulators.
5. When audio signal processing is performed, signal amplification is generally performed with the midpoint voltage between the power supply voltage and the ground potential as a reference voltage. As a voltage generating circuit for easily generating such a reference voltage, a circuit, which is provided with two voltage dividing resistors having the same resistance connected in series between the power supply terminal and the ground terminal for dividing the power supply voltage, is widely used. A patent document 5 discloses a related art.
[Patent Document 1] Japanese Patent Application Laid-open No. H9-69729
[Patent Document 2] Japanese Patent Application Laid-open No. 2005-102321
[Patent Document 3] Japanese Patent Application Laid-open No. H5-175922
[Patent Document 4] Japanese Patent Application Laid-open No. H5-130059
[Patent Document 5] Japanese Patent Application Laid-open No. H9-190698
1. However, conventionally, since the modulating signal is multiplexed (superimposed) with a high-frequency signal using a coupling capacitor and a resistor, there has been a problem that the impedance of the loop filter of the PLL has an influence on the input signal as the modulating signal. When the modulating signal is influenced by the loop filter, the modulating signal of the high-frequency range is attenuated and therefore, in the case where an audio signal is modulated, for example, distortion occurs in the signal. Further, the signal output from the loop filter may be leaked to the side of the circuit that generates the modulating signal.
2. As disclosed in the above described patent documents, in the stereo modulator, a sum signal of the L-channel signal and the R-channel signal (also referred to a main channel signal) and a difference signal of the L-signal and the R-signal (also referred to as a subchannel signal) are generated, and a subcarrier wave is amplitude-modulated using the subchannel signal. Then, the amplitude-modulated subcarrier wave and the main channel signal are multiplexed, a pilot signal is further multiplexed, and thereby, a stereo composite signal is generated.
As disclosed in patent document 2, in the FM stereo transmitter, the stereo composite signal output from the stereo modulator is input to the frequency modulator placed downstream. Here, typically, the subcarrier wave having a preset frequency of 38 kHz and the pilot signal having a preset frequency of 19 kHz are delayed differently by the VCO and the loop filter contained in the frequency modulator and filters provided between the respective circuit blocks, and therefore, there has been a problem that the separation characteristics are deteriorated.
In order to solve the problem, conventionally, phase delay and phase-lead may be corrected using a resistor and a capacitor, however, since a very large capacitor is required for changing the phase in the frequency range of 19 kHz and 38 kHz, it has been difficult to form as an IC. Further, if mounted as chip components, there has been a problem that adjustment with respect to each set is difficult.
3. The input-output characteristic, i.e., the relationship between the input voltage and the oscillation frequency of the VCO used for the direct-modulation frequency modulator is often nonlinear and the modulation sensitivity characteristic changes according to the frequency. As a result, when the amplitude of the modulating signal input to the frequency modulator is constant, the modulation degree may change according to the change of the frequency of the modulated signal and distortion may occur.
4. In the FM transmitter, the modulated signal output from the frequency modulator is amplified by a power amplifier and transmitted from an antenna to a receiver. Since the gain of the power amplifier has a frequency characteristic, in a conventional FM transmitter, there has been a problem that the transmission power is not constant depending on the frequency of the modulated signal (modulation frequency).
5. In this case, a capacitor having a large capacitance is generally provided between a voltage dividing point of the resistance and the ground so that the reference voltage may not vary even when the power supply voltage varies. The capacitor and the voltage dividing resistor configure a time constant circuit. In order to reduce the power consumption by the voltage dividing resistor, its resistance is set larger, accordingly the time constant of the time constant circuit formed with the capacitor and the voltage dividing resistor become very large. As a result, there has been a problem that the time, for charging the capacitor when the power supply is turned on, becomes long and then the time for raising the reference voltage becomes long.